In the field of radially extensible vane compressors, vane wear is a common concern. Due to the great centrifugal forces acting on the vanes, and the great velocity differences between the vanes and the internal walls of the compressor housing that the vanes contact, vanes wear long before the other components of the compressor. Timing of vane replacement has a significant effect on operating costs of the vane compressor. In a typical chemical plant application, the entire process must be stopped for vanes to be replaced, the compressor must be disassembled, and the vanes checked and replaced as necessary. This results in significant downtime for the chemical processing plant, and typically all such repairs are made as rapidly as possible to allow the equipment to be reassembled and brought back up on line as soon as possible. Measuring vane wear typically requires the use of dial indicators, and other measuring instruments that may be easily misread or broken in the haste of checking and repairing the compressor. Alternatively, vane wear may be measured without significantly disassembling the compressor by removing an inspection port of the vane compressor, "bumping" the rotor until it is in the proper rotational position for each vane, and measuring the distance between the edge of the inspection port and the outermost wearing edge of the vane. As the vane wears, this distance will become greater and greater until it finally indicates the need for repair. This process has the disadvantage of requiring careful positioning of the rotor and manipulation of the depth gauge, and has the further disadvantage of requiring subsequent disassembly if the vanes are found to be in need of repair. What is needed is an improved method and apparatus for checking vane wear in a radially extensible vane compressor that is more accurate, that does not require expensive measuring instruments, and is less likely to be misread.